4. Set Up The Head Node

So let's get "wolfing." Choose the most powerful box to be the head
node. Install Linux there and choose every package you want. The only
requirement is that you choose "Network Servers" [in Red Hat terminology]
because you need to have NFS and ssh. That's all you need. In my case, I
was going to do development of the Beowulf application, so I added X and C
development.

It is my experience that you do not actually need NFS, but I found
it invaluable for copying files between nodes, and for automating the
install process. Later in this document I will describe how you can run a
simple Beowulf application without the use of NFS, but a more complex
application may use NFS or actually depend upon it.

Those of you researching Beowulf systems will also know how you can
have a second network card on the head node so you can access it from the
outside world. This is not required for the operation of a cluster.

I learned the hard way: use a password that obeys the strong
password constraints for your Linux distribution. I used an easily typed
password like "a" for my user, and the whole thing did not work. When I
changed my password to a legal password, with mixed numbers, characters,
upper and lower case, it worked.

If you use lam as your message passing interface, you will read in
the manual to turn OFF the firewalls, because they use random port numbers
to communicate between nodes. Here is a rule: If the manual tells you to
do something, DO IT! The lam manual also tells you to run as a non-root
user. Make the same user for every box. Build every box on the cluster
with that same user and password. I named that non root user "wolf".

4.1. Hosts

First we modify /etc/hosts. In it, you will see the comments
telling you to leave the "localhost" line alone. Ignore that advice and
change it to not include the name of your box in the loopback
address.

Modify the line that says:

127.0.0.1 wolf00 localhost.localdomain localhost

...to now say:

127.0.0.1 localhost.localdomain localhost

Then add all the boxes you want on your cluster. Note: This is not
required for the operation of a Beowulf cluster; only convenient, so
that you may type a simple "wolf01" when you refer to a box on your
cluster instead of the more tedious 192.168.0.101:

4.4. IP Addresses

My network is 192.168.0.nnn because it is one of the "private" IP
ranges. Thomas Sterling talks about it on page 106 of his book. It is
inside my firewall, and works just fine.

My head node, which I call "wolf00" is 192.168.0.100, and every
other node is named "wolfnn", with an ip of 192.168.0.100 + nn. I am
following the sage advice of many of the web pages out there, and
setting myself up for an easier task of scaling up my cluster.

...And, during startup, I saw some services that I know I don't
want, and in my opinion, could be removed. You may add or remove others
that suit your needs; just include the ones shown above.

chkconfig -del atd
chkconfig -del rsh
chkconfig -del sendmail

4.6. SSH

To be responsible, we make ssh work. While logged in as root, you
must modify the /etc/ssh/sshd_config file. The lines:

#RSAAuthentication yes
#AuthorizedKeysFile .ssh/authorized_keys

...are commented out, so uncomment them [remove the #].

Reboot, and log back in as wolf, because the operation of your
cluster will always be done from the user "wolf". Also, the hosts file
modifications done earlier must take effect. Logging out and back in
will not do this. To be sure, reboot the box, and make sure your prompt
shows hostname "wolf00".

To generate your public and private SSH keys, do this:

ssh-keygen -b 1024 -f ~/.ssh/id_rsa -t rsa -N ""

...and it will display a few messages, and tell you that it created
the public / private key pair. You will see these files, id_rsa and
id_rsa.pub, in the /home/wolf/.ssh directory.

Copy the id_rsa.pub file into a file called "authorized_keys"
right there in the .ssh directory. We will be using this file later.
Verify that the contents of this file show the hostname [the reason we
rebooted the box]. Modify the security on the files, and the
directory:

chmod 644 ~/.ssh/auth*
chmod 755 ~/.ssh

According to the LAM user group, only the head node needs to log
on to the slave nodes; not the other way around. Therefore when we copy
the public key files, we only copy the head node's key file to each
slave node, and set up the agent on the head node. This is MUCH easier
than copying all authorized_keys files to all nodes. I will describe
this in more detail later.

Note: I only am documenting what the LAM distribution of the
message passing interface requires; if you chose another message passing
interface to build your cluster, your requirements may differ.

At the end of /home/wolf/.bash_profile, add the following
statements [again this is lam-specific; your requirements may
vary]:

export LAMRSH='ssh -x'
ssh-agent sh -c 'ssh-add && bash'

4.7. MPI

Lastly, put your message passing interface on the box. As stated
in 1.2 Requirements, I used lam. You can get lam from here:

...but you can use any other message passing interface or parallel
virtual machine software you want. Again, I am showing you what worked
for me.

You can either build LAM from the supplied source, or use their
precompiled RPM package. It is not in the scope of this document to
describe that; I just got the source and followed the directions, and in
another experiment I installed their rpm. Both of them worked fine.
Remember the whole reason we are doing this is to learn; go forth and
learn.

You may also read more documentation regarding LAM and other
message passing interface software here.